1. Field of the Invention
The invention relates generally to camera viewfinders for viewing and framing the subject to be photographed, and particularly to a bright-line viewfinder, sometimes called a brillant-frame finder. More specifically, the invention relates to a viewfinder having a near-subject (or parallax correction) frame and a distant-subject frame visible in the finder field to frame respective subjects closer to and farther from the camera than a predetermined distance, e.g., four feet.
2. Description of the Prior Art
When taking a picture it is necessary to know not only in what direction to aim the camera but also how much of the scene will be included in the picture. For this reason practically every modern camera has a viewfinder of some kind.
Most modern cameras include an optical viewfinder which comprises a negative finder lens, and a positive eyelens mounted behind the negative lens, for viewing an erect image of the subject to be photographed. The limits of the field of view in the finder may be masked on the negative lens, and the scene is viewed through the finder with the camera held at eye level. In one popular viewfinder, known as the Albada finder, the concave curve on the negative lens is made partially reflective with a metalized coating, such as aluminum. The coating reflects a white frame defined by a series of rectangularly arranged slots in a black mask which surrounds the eyelens. The photographer sees the white frame as suspended in space superimposed over the scene viewed through the finder. The white frame surrounds the portion of the field of view in the finder which will be projected on the film.
A more recent viewfinder somewhat similar to the Albada finder is the bright-line or brilliant-frame finder. In this viewfinder, a second window is provided adjacent the finder window. The second window is covered with a diffusing screen, and the rectangular series of slots is cut into a masking plate situated behind the screen. A prism or mirror behind the masking plate reflects a bright-line frame, formed by ambient light in the slots, onto a semi-reflecting mirror in the viewing path of the finder, and from there into the positive eyelens. This arrangement provides a brighter frame for the viewed subject as compared to the Albada finder.
When the viewfinder, even though pointing in the same direction as the camera lens, takes in a field of view that is displaced relative to the view projected on the film, it suffers from a framing error. This is commonly referred to as the parallax error. It arises from the difference in viewpoint between the viewfinder and the camera lens, which may be one to two inches apart. With a distant subject which may have a subject-field of many feet or even yards, an inch or two is obviously insignificant. At close range, however, for instance with the camera lens focused at three feet, the subject-field may only be two to three feet wide. There, a couple of inches lost at the top of the scene may result in the cutting off of important subject matter. For this reason many cameras include some means for correcting for parallax in the finder.
There are various ways of compensating for the framing error due to the finder parallax. The simplest is to cut down the finder field so that it always takes in less than the film. With distant subjects, the view projected on the film will then include more at the top than in the finder field, while at near distances there will be more recorded at the bottom of the picture than in the finder field. This solution is less than satisfactory since the photographer cannot be sure of the actual extent to which the subject-field will be recorded on the film.
Bright-line finders usually have one of two types of correction for parallax. The simplest is a second bright-line frame that defines a subject-field which includes slightly more at the bottom and one side, with a corresponding reduction at the top and the other side, as compared to the subject-field defined by the normal bright-line frame. Thus, the second bright-line frame serves as a parallax correction frame for framing close-up subjects nearer to the camera lens than say four feet, and the normal bright-line frame is used to frame other subjects beyond that distance. In operation, the normal frame is used far more frequently than the parallax correction frame.
Often, in bright-line finders having a parallax correction frame for framing close-up subjects and a normal frame for framing other subjects, the two frames are illuminated by ambient light in the viewing path, and therefore are both visible in the finder field at the same time. This may cause the photographer to confuse the parallax correction frame with the normal frame, or vice versa, especially when picture-taking must be done quickly.
More advanced cameras have a bright-line finder in which a single bright-line frame is alternatively located in a normal position for most subjects and in a parallax compensation position for close-up subjects. Thus, only a single bright-line frame is ever visible in the finder. Typically, the mask for producing the bright-line frame is coupled to the focusing mechanism for the camera lens. This allows the bright-line frame to be shifted in the finder field to the appropriate position in accordance with the focus position of the camera lens. However, this arrangement is relatively complex from a mechanical standpoint, and therefore increases the manufacturing cost of the camera and the possible risk of a break-down.